The invention concerns processes for convective movement of static or flowing fluids in Microsystems, particularly for electroconvective or thermoconvective mixing of the fluids, and devices for implementation of the processes, particularly electrode arrangements in Microsystems for triggering convective fluid movements.
In numerous technical fields, particularly in chemical technology, the task of circulating or stirring a fluid or mingling or mixing several fluids frequently arises. For this purpose, fluid currents are generated which are, for example, mechanically circulated by means of mechanical barriers and/or actively movable elements. During the turbulent swirling, the fluid(s) is/are mutually interspersed. The Reynolds number of a fluid is important for the effectiveness of its circulation in a channel or container structure. For the mechanical mixing of fluids in a container structure, this Reynolds number must have a value over 1000. These values can only achieved in macroscopic systems, as the following estimation shows.
The Reynolds number of a channel can be estimated according to Re =(xcfx81xc2x7Uxc2x7L)/xcex7, with xcfx81 being the density of the fluid, xcex7 being the dynamic viscosity of the fluid, U being the flow speed, and L being a characteristic channel dimension (e.g. radius of the channel cross-section). An aqueous solution with kinematic viscosity xcexd=xcex7/xcfx81=1.6xc2x710xe2x88x922 cm2/s which flows through a channel with a radius r=25 xcexcm at a speed U=500 xcexcm/s, would, for example, result in a Reynolds number Re≈0.008, which is far below the required value of 1000 mentioned above. The mingling of fluids due to obstacles in the flow by means of flow mechanics is therefore restricted to macroscopic systems. The use of actively moving elements for the circulation of fluid is also restricted to macroscopic systems, because movable elements are subject to breakdown and can easily cause blockages or flow impediments in miniaturized systems.
The measurement and/or analysis systems of many biological, medical, and chemical technology applications have been miniaturized in the last decade for reasons of cost and resources and to achieve highly specific analyses. The problem of fluid circulation in Microsystems has, however, not yet been solved. Due to the low Reynolds number, even with flowing around, e.g., barriers which cross in a meander shape or sharp-edged flow obstructions, no turbulent flow can result. Therefore, if two fluids are introduced into a miniaturized channel (typical cross-section: 1 mm2), no mixing of the fluids will result except through diffusion, even during flow through a channel length of several millimeters.
A generally known attempt to cause the circulation of flowing fluids in Microsystems consists of splitting a channel into a number of narrower channels and rejoining them with a changed relative arrangement. It is true that no movable parts are used hereby. However, the narrowed channels have a characteristic diameter which is smaller than the initial channel by a factor of 10 to 40. The flow resistance thereby increases and an acute danger of blockage arises. Usage for suspensions which contain particles such as biological cells or microbeads is excluded. In addition, only a quasi-blending corresponding to the number and rearrangement of the narrowed channels results.
Pumping fluids on the basis of electrohydrodynamic effects is also known. Traveling electric fields are generated in fluid channels with electrode systems which are affixed to opposing channel walls over the entire length of the channel. In combination with a temperature gradient which is directed from one of the electrode systems to the opposite electrode system, so-called electroconvection occurs, which effects a stationary fluid transport in the channel. These types of systems are, for example, described as traveling wave pumps or electrohydrodynamic pumps by J. R. Melcher et al. in xe2x80x9cThe Physics of Fluidsxe2x80x9d, vol. 10, 1967, p. 1178 et seq. The mechanical fluid propulsion is caused in such a way that, due to the temperature gradients in the fluid, conductivity and/or dielectric constant gradients arise. Volume charges are thereby generated which exercise a propulsive force on the fluid by interacting with the traveling electric field.
The system described by J. R. Melcher et al. is a macroscopic system with a channel length of approximately 1 m and a typical channel cross-section of approximately 3 cm. It serves exclusively for the investigation of electrode convection and, due to the expensive measures for the production of the temperature gradients and for driving the electrodes over the entire length of the channel, does not allow for practical use.
Miniaturized traveling wave pumps are described by Fuhr et al. in xe2x80x9cMEMS 92xe2x80x9d, 1992, p. 25. The implementation of the traveling wave principle in Microsystems has, however, not yet found practical application, because there are significantly simpler possibilities for fluid transport in microchannels and a contribution to the problems described above of fluid circulation in Microsystems has not been provided. Fluid circulation would specifically mean that the sum of the fluids circulating in a region of the Microsystems is zero. The conventional traveling wave pumps, however, always provide a net flow of solution. Directed pumping along the channel direction occurs in the microsystem. Mixing of fluids is not possible with the conventional traveling wave pumps.
It is the object of the invention to provide improved processes for convective movement of fluids in Microsystems, with which circulation or blending of fluids in microchannels is made possible without moving parts and without narrowing the channels, and with any desired channel cross-section. In particular, the object is to provide a process for effective fluid mixing in Microsystems which can also be used with suspensions containing microparticles. It is also the object of the invention to indicate devices for implementation of the process mentioned, particularly miniaturized fluid mixers.
According to a first aspect of the invention in particular, a new process for convective fluid movement in microsystems is created in which one or more fluids in the microsystem are subjected to traveling electric fields, alternating fields, or electrical field gradients having an alignment which deviates from a flow direction of the fluid in the microsystem and/or a preferred lengthwise alignment of a section of the microsystem (e.g. channel section). The alignment of the alternating fields (preferred direction of the field-generating electrodes), of the traveling electrical fields (direction of travel), or of the field gradients will be generally referred to in the following as the field direction. According to the invention, the field direction runs, e.g., perpendicular to the flow direction of the fluid and/or perpendicular to the channel alignment.
The convective fluid movement can be generated both in flowing fluids (transverse to the flow direction) and in static fluid volumes (e.g. in a closed part of the microsystem). The convective fluid movement is characterized by a closed fluid circulation. The sum of the flows caused in the region of the field gradients implemented according to the invention is zero. Thus, for example, flow loops are generated transverse to the direction of the channel which cause a swirling and mixing of the fluids involved. This is a surprising result, after free mixing of fluids in microsystems was thought to be impossible due to the reasons of flow mechanics described above.
The convective fluid movement is triggered according to the following principles. At the interface between two fluids with different dielectric constants (or conductivities), the field gradients lead to the appearance of polarization and force effects which lead to blending at the interface and at each new interface. In fluids or fluid mixtures with sufficient anisotropy of the dielectric properties or polarization properties, blending is achieved by the electrical field gradients alone. If the fluid is isotropic, electrical anisotropy must be artificially achieved by formation of a thermal gradient. The action of the thermal gradients will be explained with the following image. As the temperature changes in a fluid which is initially isotropic, gradients of the dielectric properties or polarization properties corresponding to the temperature gradients are also formed. The fluid can be viewed as a lamination of many dielectrically different fluids. The effects mentioned for the anisotropic fluids arise at the interfaces between the layers. The appearance of electrical polarization leads to mingling of the fluid.
According to a preferred embodiment of the invention, the formation of a thermal field gradient parallel to the field direction thereby occurs simultaneously with the generation of the electric fields. The thermal gradient is required in order to generate the anisotropy in the fluid which leads, together with the electrical field, to the fluid propulsion. In contrast to the conventional traveling wave pumps, a thermal gradient with a temperature difference between opposing channel walls of 0.5xc2x0 to 1xc2x0 is sufficient to generate the fluid circulation or cross-current according to the invention. A particular advantage of the invention is that this type of temperature difference can be achieved just by the application of electrical voltages for generating the electric fields to the electrode arrangements, so that separate generation of an external thermal gradient is not urgently necessary.
If the thermal gradient is externally generated, this is preferably done with optical irradiation. The region of the microsystem of interest, in which the electrical field gradients are implemented, is radiated with light of a suitable wavelength which is well absorbed in the respective fluid. The irradiation is preferably performed with a focused laser beam which, depending on the application, can be coupled from any desired side of the microsystem through transparent wall regions or using light guides. So-called xe2x80x9chot spotsxe2x80x9d are formed by the optically induced increase in temperature, which work together particularly effectively with the electrical field gradients to generate the convective fluid movement.
According to the invention, there is a predetermined difference in angle between the field direction and the direction of the current flow direction of the fluid and/or the flow direction before or after realization of the process. In the following, the concept of flow direction is generally used for the alignment of the fluid flow or for the alignment of the microsystem region in which the fluid flows. The angle between the field direction and the flow direction is preferably in the range from 60xc2x0 to 120xc2x0. For values above 90xc2x0, this means that the field direction has a component which is opposite to the flow direction.
According to a further aspect of the invention, a fluid microsystem is provided having structures which are set up for fluid conduction or fluid accomodation and, in at least one predetermined section (swirling section), have an electrode arrangement for the formation of the traveling electric fields, electrical field gradients, or AC voltages corresponding to the desired field direction. The structures in the microsystem preferably have a characteristic cross-sectional dimension of less than 150 xcexcm. Typically, a structure is implemented as a microchannel with a cross-sectional area of approximately 1 mm2 (or less), e.g. cross-sectional dimensions of 100 xcexcmxc2x7100 xcexcm or less. The provision of swirling sections is possible in all types of Microsystems known per se. The application of electrode arrangements according to the invention to straight channels is preferred.
The subject of the invention is also an electrode arrangement affixed to at least one wall of a microchannel for the implementation of the field effects mentioned in a field direction deviating from the channel alignment. Because the thermal gradients are generated in the field direction simultaneously with the electrical driving, the electrode arrangement consists of electrode elements which have an asymmetrical or irregular shape relative to the field direction. This applies at least for the embodiment in which the electrical fields comprise electrical field gradients or AC voltages. If traveling electrical fields are used, asymmetry of the electrode elements is not necessary, because then the thermal field gradient can also be generated through delayed driving of the electrode elements.
The invention has the following advantages. For the first time, the convective fluid movement for the generation of fluid cross-currents and/or swirls in microchannels is realized. The electrode arrangements according to the invention have a simple and compact design. It is therefore sufficient if the swirling sections in the microsystem have a relatively small extension in the lengthwise direction of the channel, in the range approximately between a fifth of and one channel cross-sectional dimension. The fluid swirling according to the invention can be realized in both still and flowing fluids. An effective temperature gradient can be easily generated electrically with the electrode arrangements. The application of an additional, external temperature gradient is possible, but not urgently required. The invention is easily compatible with other microstructure technologies. Thus, the electrode arrangements can consist of electrodes which are essentially designed like electrodes for the generation of field barriers for dielectrophoretic manipulation. According to the invention, no moving parts are required.